New tutorial for: Retriever training

[mkkuemmel]

Describe the tutorial you would like to see here

While we provide tutorials on GPL and DPR-training (-> unsupervised and supervised Retriever training), there is no tutorial on how to utilize the train method of the EmbeddingRetriever. I think for many users, training the EmbeddingRetriever with their annotated data could be the most straightforward idea on how to improve performance (instead of using GPL or switching to DPR).

[x] I've checked the existing tutorials

[bilgeyucel]

We plan to deprecate DPR starting from v1.16, so I'll start working on this new tutorial as an alternative